The subject disclosure relates to superconducting devices, and more specifically, to fabricating a vertical transmon qubit device with microstrip waveguides using a silicon-on-metal (SOM) substrate.
Quantum computing is generally the use of quantum-mechanical phenomena for the purpose of performing computing and information processing functions. Quantum computing can be viewed in contrast to classical computing, which generally operates on binary values with transistors. That is, while classical computers can operate on bit values that are either 0 or 1, quantum computers operate on quantum bits that comprise superpositions of both 0 and 1, can entangle multiple quantum bits, and use interference.
Quantum computing hardware can be different from classical computing hardware. In particular, superconducting quantum circuits generally rely on Josephson junctions, which can be fabricated in a semiconductor device. A Josephson junction generally manifests the Josephson effect of a supercurrent, where current can flow indefinitely across a Josephson junction without an applied voltage. A Josephson junction can be created by weakly coupling two superconductors (sometimes called a superconducting material; a material that conducts electricity without resistance), for example, by a tunnel barrier as described below.
One way in which a Josephson junction can be used in quantum computing is by using the Josephson junction to form a qubit. A Josephson junction can be used to form a qubit by arranging the Josephson junction in parallel with a shunting capacitor. This arrangement of a Josephson junction in parallel with a shunting capacitor is sometimes referred to as a transmon (which is a shortened version of the phrase transmission line shunted plasma oscillation qubit) in the special case where the shunting capacitor has a large capacitance such that the typical ratio of the Josephson energy to the charging energy in the qubit is larger than 10. It is generally understood by those skilled in the art that smaller ratios of the Josephson and the charging energies may not be called a transmon, but for the purposes of this invention, a transmon may designate any arrangement of a Josephson junction in parallel with a shunting capacitor.
A transmon generally has a reduced sensitivity to charge noise compared to some other types of qubits. A mechanism by which a transmon can reduce sensitivity to charge noise by increasing a ratio of Josephson energy to charging energy.
A problem with some prior art transmon qubits is that they occupy a relatively large amount of space. Specifically, a planar capacitor used in some transmon qubits occupies a large area. The compactness of such transmon qubits is limited by both surface and dielectric loss.
Then, there are also problems with some types of prior art Josephson junctions, as applied to producing transmons from these Josephson junctions. A type of Josephson junction with both low loss and low critical current can be made from shadow evaporated aluminum, aluminum oxide, and aluminum (Al-AlOx-Al). However, a problem with such a Josephson junction is that, once the Josephson junction is formed, the resulting device is subject to low-temperature and process constraints.
Additionally, a transmon qubit that utilizes a vertical Josephson junction generally must be designed differently in order to couple to other microwave resonators and/or circuits. Encapsulating superconducting qubit components in the prior art is generally avoided because of a loss associated with deposited dielectrics.
For a vertical transmon qubit on SOM, while a coplanar waveguide can work for circuits on a bottom superconductor layer, this approach may not work on the top superconductor layer, because the bottom metal layer is too close to this top layer in a SOM substrate. Thus, in other transmon qubits, only one level of metal is used for defining resonators on, and in particular, the bottom metal layer is used for defining coupling circuitry.
Additionally, in other approaches for transmon qubits, to form a circuit or to create a resonator, the buried metal needs to be accessed by etching down to the buried metal of the SOM.